Neutron sources are always accompanied by Gamma and X-ray radiation, either as a result of the primary reaction or emitted by the materials surrounding the primary neutron source. Thermalized fast neutrons when captured are usually accompanied by X and/or Gamma rays. In addition there is the background radiation that often is higher than the neutron flux. Thus the main challenge of neutron detection is the elimination or differentiation from Gamma and X-rays.
Low energy Gamma rays interact with matter mainly by one of the 3 effects, the photoelectric, the Compton or the pair production effects.
The energy transfer from a hard photon (hv) to an atomic electron (e) in the photoelectric effect is given by Ee=hv−Eb where Eb is the binding energy of the electrons of the stopping material. The atom excited by the stripping of one of its electrons, returns to its stable state by emitting one or more X-rays whose energies are determined by its discrete energy levels and denoted accordingly as the M, L or K X-rays.
In the Compton effect which is dominant at medium energies and low Z elements, the incoming photons are scattered by the electrons of the stopping material, imparting them part of their energy. The energy of the scattered Gamma ray (1) when the electron is considered to be at rest, is given byE1=hv′=mec2/[1+cos θ+(mec2/E0)]or cos θ=1−(mec2/Ee)+(mec2/E1+Ee)the recoil electron's energy is given byEe=hv′=[(hv)2/mec2(1−cos θ)]/[1+(hv/mec2)(1−cos θ)]
The maximal energy of the recoil electron is therefore at Eemax=E/(1+mec2/2E)
The recoil angle φ of the electron, relative to the direction of the impinging Gamma ray, is given bycot φ=1+(hv/mec2)tan(θ/2)
It is important to note that following the momentum equalities, the incoming Gamma ray, the scattered Gamma ray and the recoil electron are all on the same plane.
The differential cross section of the Compton Scattering for unpolarized photons is given by the Klein-Nishina equation:[dσ/dΩ]=(re2/2)(v′/v)2[(v/v′)+(v′/v)−sin2θ]where re=(e2/mec2) is the “classical” radius of the electron equal to 2.82 10−13 cm.
This equation which assumes scattering by free electrons, has to be modified by a form factor S(k,k′) at energies where the binding energies of the electrons become important, as compared with the energy of the Gamma ray, causing the angular distribution in the forward direction to be suppressed. The binding energies of the electrons in a plastic are low, 13.5981 eV for the electron in a H atom and in a carbon atom 288 eV for the inner is electron, 16.6 eV for the 2s electrons and 11.3 eV for the 2p electrons. Thus in a Compton scattering of a 140 keV Gamma ray by a plastic scintillator, the highest energy of scattering K electrons is 0.2% and may be viewed practically as at rest; the resultant X-Ray in practice is unobservable.
In the pair production effect the two 511 keV hard photons generated by the annihilation of the positron, again interact with the stopping material through the Compton or Photoelectric effects and cause ejection of electrons and their eventual absorption, as explained above.
As described above a Gamma ray will interact with a scintillator through one or several of the Photoelectric, Compton or Pair production processes. At the end of each of the processes the energy is transmitted to an electron that spends this energy exciting successive atoms of the material The electrons so produced by the three processes loose energy mainly by Coulomb scattering until they eventually stop. If the stopping medium is a scintillator the excited electrons and the holes along the track form excitons that drift in the scintillator until they excite a color center which when deexcited emit low energy photons in the visible range. If the stopping medium is transparent to these photons, they can emerge from it and be detected by a photon detector such as a photomultiplier tube or a photo-diode.
The loss of energy (dE/dx) by an ionizing particle, other than electrons, in a material of atomic number Z is given by the Bethe-Bloch equation−(dE/dx)=(4π/mec2)(nz2/β2)(e2/4π∈0)2[ln {2mec2β2/I(1−β2)}−β2]where β=v/c, E=energy of the particle, z=the charge of the particle, and I=the mean excitation energy of the target. n=the electron density of the target given by (NZμ/A) where N is the Avogadro number and Z, ρ and A are the atomic number, the density and the mass number of the target respectively. The stopping power and Range of protons and alpha particles is tabulated in the databases PSTAR, and ASTAR of NIST (National Institute of Science and Technology), according to methods described in ICRU (International Commission on Radiation Units) Reports 37 and 49. The stopping power of electrons is adequately described by Coulomb scattering and tabulated in ESTAR tables of MST and ICRU-37. In low Z materials and low energies in the 0.01≦E≦2.5 MeV an electron's range R (expressed in g/cm2 units) may be approximated by the empirical formulaR=0.412E1.27−0.0954lnE where E is the kinetic energy of the electron in units of MeV
The most prevalent thermal neutron detectors consist of pressurized He3 and BF3 gas detectors that detect the ionizing particles formed in the reactions He3+n→H3+p and 10B+n→Li(0.84 MeV)+4He(1.47 MeV)+γ(0.48 MeV)
Solid-state semiconductor neutron detectors—using various semiconductors incorporating Boron, Cadmium, Gadolinium and Lithium or deposited as a conversion layer on top of a Si diode 215 have been constructed. Notwithstanding the technological problems associated with the specific semiconductors, such p-n diodes are inherently small; consequently they have low efficiencies to detect neutrons.
Neutron detectors may be constructed by using photomultipliers or photodiodes to detect the scintillations generated within a liquid scintillator loaded with Boron, when a thermal neutron is catured. Liquid scintillator loaded with up to 10% Boron in weight, are commercially available from Saint-Gobain (BC-454).
Neutron Detectors formed in polyvinyl toluene and silicone rubber-based solid scintillators loaded with 1% gadolinium are described by Zane W Bell from Oak ridge National Labs. P. K. Lightfoot et at have shown liquid scintillators based on α-hydroxytoluene loaded with up to 10% by weight of gadolinium.
Gadolinium Pyrosilicate (Gd2SiO5) and Gadox (GSO) scintillators can be used as very efficient thermal neutron detectors, but are also sensitive to Gamma rays.
N. Mascarenhas et al. from Sandia have shown a Neutron Detector composed of 500μ square plastic scintillator fibers detecting the recoil protons of high energy fast neutrons. However such thick fibers cannot minimize the sensitivity to Gamma radiation and the detector built with such thick fibers cannot be inherently directional, without imaging the track of the recoil proton.
Neal et al. from PNNL/Battelle have built fast/thermal neutron detectors using glass scintillator fibers containing Li6 surrounded by plastic scintillator acting both as moderator and detector of the recoil protons created after a fast neutron scattering and the thermal neutrons following the reaction Li6+ n→He4(2.05 MeV)+H3(2.73 MeV).
However to the best of our knowledge there are no neutron detectors built out of scintillation fibers whose thicknesses are less than 100μ, the track lengths of the protons, alphas and tritons produced by neutrons interacting with hydrogenous scintillators or scintillators loaded with Li or Boron